Chemical looping combustion (CLC) is generally characterized by 2 fundamental stages. In the first stage a solid material, typically desirably sized solid particles of metal oxide, are contacted with a carbon containing fuel such that the metal oxide gives up its oxygen to the fuel, forming carbon dioxide and a reduced state material. Depending on the materials and fuels, this step may generate or consume heat. In the second stage, the reduced state material is regenerated in the absence of fuel by exposing it to air where the oxygen oxidizes the reduced state material to regenerate a fully oxidized species. In conventional CLC, this step always results in the release of heat. The regenerated metal oxide is then available to repeat the cycle with more fuel. Typical metal oxides used for CLC include nickel oxide, calcium oxide, iron oxide, copper oxide, manganese oxide, and cobalt oxide.
Traditionally, a CLC system employs a dual fluidized bed system (circulating fluidized bed process) where a metal oxide is employed as a bed material providing the oxygen for combustion in the fuel reactor. The reduced metal is then transferred to the second bed (air reactor) and re-oxidized before being reintroduced back to the fuel reactor completing the loop. Isolation of the fuel from air reduces the number of discrete process steps needed to capture the CO2 generated from fuel combustion.
Many methods have been developed for purposes of CO2 capture: amine scrubbing, oxy-combustion, and/or pre-combustion decarbonization. In amine scrubbing, flue gas is typically treated with an organic amine that selectively traps the CO2 then the CO2 is subsequently released in pure form as the amine is regenerated. Oxy-combustion uses purified oxygen during combustion to generate a flue gas that is predominantly CO2. Alternatively, pre-combustion decarbonization also called gasification, when used for CO2 capture, converts the fuel to a mixture of predominantly CO2 and H2. The CO2 can be separated from the H2 prior to combustion of the H2, thus resulting in only H2O being produced from combustion. Unfortunately these clean combustion methods require large supplemental energy for CO2 capture greatly reducing their overall combustion efficiency. CLC generates clean energy from a carbonaceous fuel without an air separation unit, without a thermal regeneration step, and without the need to separate CO2 from H2, all of which require both expensive capital and require large amounts of electrical, thermal and/or mechanical energy.
Prior research has shown that CLC can be used to generate power. Ishida, et al., U.S. Pat. No. 5,447,024, use a metal oxide (MO) oxygen carrier including nickel oxide (NiO), yttrium-stabilized zirconium (ZrO2), as well as iron (Fe), copper (Cu) and manganese (Mn) oxides to combust fuel (RH) including methane (CH4) while moistened air is used to regenerate the metal oxide carrier. This process uses low temperature fuel combustion and generates heat when the MO is regenerated with moistened air. Van Harderveld, U.S. Pat. No. 6,214,305, oxidize liquid and solid contaminants in diesel exhaust by passing contaminated exhaust gas through a particulate separator with serially-arranged catalytic plates arranged so that there is no net gas flow in the space between two adjacent vertical plates. The catalytic plates contain a mixture of metal salts and metal oxides including vanadium, molybdenum, molybdenum oxide, iron, platinum, palladium and alkali metals that oxidize soot. Although van Harderveld discloses the metal salt and metal oxide mixtures may become molten at reaction temperatures, a solid metal or ceramic support is required to maintain catalytic activity for these metal mixtures with low melting points. Lyon, U.S. Pat. No. 5,827,496, uses cyclic exposure of a catalytic reaction bed including metal oxides to a reducing gas and molecular oxygen to reduce and oxidize a fuel on the combustion catalyst. Lyon uses silver/silver oxide, copper/copper oxide, iron/iron oxide, cobalt/cobalt oxide, tungsten/tungsten oxide, manganese/manganese oxide, molybdenum/molybdenum oxide, strontium sulfide/strontium sulfate, barium sulfide/barium sulfate, and mixtures thereof for a catalytic reaction bed.
Yao and associates, (Yao, 2008) use a transition-metal carbide including Cr2O3, MoO3, V2O5, Nb2O5 and TiO2 with cementing-metal oxides CO3O4 and NiO, and carbon black as composite powders such as Cr3C2—Co, Mo2C—Co, VC—Co, NbC—Co and Tic-Ni, for direct reduction and carburization. CO3O4 and NiO improved carbothermal properties for a direct reduction and carburization process.
The main problem with solid oxygen carriers is that they undergo mechanical degradation while being cycled from a reduced state to an oxidized state over and over again, from attrition, erosion, fatigue, crystalline changes, irreversible and side reactions, etc. The result is that the solid carriers turn into ever finer particles, making moving and managing solids quite difficult, and making the useful lifetime of the oxygen carriers uneconomically short. Moreover, continuously moving hot solid particles from one reactor to another is complicated. A simpler system is required that mitigates the movement of the metal/metal oxide at very high temperatures. In addition to mechanical degradation, chemical degradation of the solid oxygen carrier also occurs with each cycle, such that the fraction of the total oxygen carrier available to take part in chemical reactions continuously decreases with use. In practical applications, a small amount of the degraded oxygen carrier is continuously removed from the reaction process and a small amount of fresh oxygen carrier is continuously added, but an improved process would offer an oxygen carrier with an extended useful lifetime.